Potential impacts of brown tide, Aureococcus anophagefferens, on juvenile hard clams, Mercenaria mercenaria, in the Coastal Bays of Maryland, USA

The rate of growth of juvenile hard clams, Mercenaria mercenaria, was studied in the Coastal Bays of Maryland during an outbreak of the brown tide, Aureococcus anophagefferens. Brown tide dominated the plankton community during the month of June 2002, with cell densities at several sites reaching category 3 (>200,000 cells ml⁻¹) levels. Temperatures during the bloom were 18.6–27.5 °C. Nutrient conditions preceding and during the bloom were conducive for the proliferation of A. anophagefferens: while inorganic nitrogen and phosphorus were <1 μg at N or P l⁻¹, urea was elevated during bloom development. Organic nitrogen, phosphorus and carbon were in the range of levels observed in previous brown tide blooms and increased following the collapse of the bloom. Growth rates of juvenile clams were significantly lower during the period of the brown tide bloom than following its collapse. Growth rates of M. mercenaria were found to be negatively impacted at brown tide densities as low as 20,000 cells ml⁻¹, or category 1 levels. The low growth rates of M. mercenaria could not be explained by temperature, as the lowest growth rates were found when water temperatures were at levels previously found to be optimal for growth.     

Microbial herbivory on the brown tide alga, Aureococcus anophagefferens: results from natural ecosystems, mesocosms and laboratory experiments

Experiments were conducted with natural plankton assemblages from two areas in Great South Bay (GSB) and the Peconic Bays Estuary System, NY, to compare the rates of growth and pelagic grazing mortality of Aureococcus anophagefferens with co-occurring phytoplankton. We hypothesized that A. anophagefferens would experience low mortality rates by microbial herbivores (relative to feeding pressure on other algae) thus providing it with a competitive advantage within the phytoplankton community. In fact, substantial rates of mortality were observed in nearly every experiment in our study. However, mortality rates of A. anophagefferens were less than intrinsic growth rates of the alga during late spring and early summer in Great South Bay, resulting in positive net growth rates for the alga during that period. This timing coincided with the development of a brown tide in this estuary. Similarly, growth rates of the alga also exceeded mortality rates during bloom development in natural plankton assemblages from the Peconic Bays Estuary System held in mesocosms. In contrast to the situation for A. anophagefferens, growth rates of the total phytoplankton assemblage, and another common picoplanktonic phytoplankter (Synechococcus spp.), were frequently less than their respective mortality rates. Mortality rates of A. anophagefferens in both systems were similar to growth rates of the alga during later stages of the bloom. Laboratory studies confirmed that species of phagotrophic protists that consume A. anophagefferens (at least in culture) are present during brown tides but preference for or against the alga appears to be species-specific among phagotrophic protists. We conclude that two scenarios may explain our results: (1) protistan species capable of consuming the brown tide alga were present at low abundances during bloom initiation and thus not able to respond rapidly to increases in the intrinsic growth rate of the alga, or (2) the brown tide alga produced substance(s) that inhibited or retarded protistan grazing activities during the period of bloom initiation. The latter scenario seems less likely given that significant mortality of A. anophagefferens was measured during our field study and mesocosm experiment. However, even a minor reduction in mortality rate due to feeding selectivity among herbivores might result in a mismatch between growth and grazing of A. anophagefferens that could give rise to significant net population growth of this HAB species. Either scenario infers an important role for trophic interactions within the plankton as a factor explaining the development of brown tides in natural ecosystems.     

Assessment of brown tide blooms, caused by Aureococcus anophagefferens, and contributing factors in New Jersey coastal bays: 2000–2002

A 3 year study (2000–2002) in Barnegat Bay-Little Egg Harbor (BB/LEH), New Jersey (USA), was conducted by the New Jersey Department of Environmental Protection, Division of Science Research and Technology (DSRT) in cooperation with several partners to assess brown tide blooms in coastal waters in NJ. Water samples were collected by boat and helicopter at coastal stations from 2000 to 2002 along with field measurements. Aureococcus anophagefferens were enumerated and associated environmental factors were analyzed. A. anophagefferens abundances were classified using the Brown Tide Bloom Index and mapped, along with salinity and temperature parameters, to their geo-referenced location using the ArcView GIS. The highest A. anophagefferens abundances (>10⁶ cells ml⁻¹), including category 3 blooms (≥200,000 cells ml⁻¹) and category 2 blooms (≥35,000 to ≤200,000 cells ml⁻¹), recurred during each of the 3 years of sampling and covered significant geographic areas of the estuary, especially in Little Egg Harbor. While category 3 blooms were generally associated with warmer water temperatures (>16 °C) and higher salinity (>25–26 ppt), these factors were not sufficient alone to explain the timing or distribution of A. anophagefferens blooms. There was no significant relationship between brown tide abundances and dissolved organic nitrogen measured in 2002 but this was consistent with other studies. Extended drought conditions, with corresponding low freshwater inputs and elevated bay water salinities, occurring during this time were conducive to blooms. A. anophagefferens abundances were well above the reported levels that have been reported to cause negative impacts on shellfish. It was shown that over 50% of the submerged aquatic vegetation (SAV) habitat located in Barnegat Bay/Little Egg Harbor was categorized as having a high frequency of category 2 or 3 blooms for all 3 years.     

Stimulation of the brown tide organism, Aureococcus anophagefferens, by selective nutrient additions to in situ mesocosms

The influence of nutrient additions and sediment exchange on Aureococcus anophagefferens growth was studied using 200 l mesocosms deployed in situ at the Southampton College Marine Science Center in Long Island, New York. A. anophagefferens cell density increased in mesocosms with separate additions of the following materials: urea + glucose and desiccation-stressed Enteromorpha tissue. A decrease in A. anophagefferens cell density was observed in mesocosms with either no additions (control) or with added nitrate. A treatment containing a sediment layer exhibited an increase in average cell densities, but the increase was not statistically significant (P = 0.15). In the 9 day experiment, net growth of A. anophagefferens was only observed during the last 3 days, which corresponded to a period of high solar irradiation. Total chlorophyll concentration declined in all treatments during the first 6 days, which corresponded to relatively low daily irradiance, suggesting low-light stress on the phytoplankton assemblage during the initial phase of the experiment. During the ensuing sunny period, a 4–5-fold increase in chlorophyll concentration was observed in the nitrate and urea treatments with lesser increases in the other treatments. A. anophagefferens density increased relative to total phytoplankton biomass (Chl basis) in the urea + glucose and Enteromorpha treatments. Results are consistent with a prevailing hypothesis that organic nitrogen nutrients favor the growth of A. anophagefferens. Specifically, our evidence indicates that A. anophagefferens exhibited net population growth under organic N, but not inorganic N nutrient (specifically NO₃⁻) loading.     

Ecology of phytoplankton communities dominated by Aureococcus anophagefferens: the role of viruses, nutrients, and microzooplankton grazing

We investigated the impact of viruses, nutrient loading, and microzooplankon grazing on phytoplankton communities in two New York estuaries that hosted blooms of the brown tide alga Aureococcus anophagefferens during 2000 and 2002. The absence of a bloom at one location during 2002 allowed for the fortuitous comparison of a bloom and non-bloom year at the same location as well as a comparison of two sites experiencing bloom and non-bloom conditions during the same year. During the study, blooms were found at locations with high levels of dissolved organic nitrogen and lower nitrate concentrations compared to a non-bloom location. Experimental additions of inorganic nitrogen and phosphorus yielded growth rates within the total phytoplankton community which significantly exceeded control treatments in 83% of experiments, while A. anophagefferens experienced significantly increased growth during only 20% of experimental inorganic nutrient additions. Consistent with prior research, these results suggest brown tides are not caused by eutrophication, but instead are more likely to occur when sources of labile DOM are readily available. Microzooplankton grazing rates on the total phytoplankton community during a bloom were lower than grazing rates at a non-bloom site, and grazing rates on A. anophagefferens were lower than grazing rates on the total community on some dates, suggesting that reduced grazing mortality may also promote brown tides. Mean densities of viruses during blooms (3 × 10⁸ ml⁻¹) were elevated compared to most estuarine environments and were twice the levels found at a non-bloom site. Experimental enrichment of the natural viral densities yielded a significant increase in A. anophagefferens growth rates relative to control treatments when background levels of viruses were low (<1.7 × 10⁸ ml⁻¹), suggesting that viruses may promote bloom occurrence by regenerating DOM or altering the composition of microbial communities.     

HPLC pigment records provide evidence of past blooms of Aureococcus anophagefferens in the Coastal Bays of Maryland and Virginia, USA

Concentrations of the accessory phytoplankton pigment 19′-butanoyloxyfucoxanthin (but-fuco), derived from archived high performance liquid chromatography (HPLC) data from the Atlantic coastal bays of Maryland and Virginia (1993–1995 and 1999–2002), were used to determine the presence of Aureococcus anophagefferens at 18 stations. Paired data of direct cell counts of A. anophagefferens and but-fuco concentration data from 2000 to 2002 were linearly regressed (R² = 0.78). This regression was used to estimate historical cell densities from 1994 to 1995 and to improve the spatial resolution from 1999 to 2002. Although the HPLC method used did not permit quantification of but-fuco before 1994, the records indicate that qualitatively A. anophagefferens was present in 1993. Quantitative measurements grouped into bloom index categories showed that annually, peak densities occurred in May to July. Severe Category 3 blooms (>200,000 cells ml⁻¹) were found in 1995, 2001, and 2002. Spatially, concentrations of but-fuco were higher in the northern extent of the study region than in the lower Chincoteague Bay, and along the western shore of Chincoteague Bay than on the eastern side. On an interannual basis, it appeared that A. anophagefferens became more geographically widespread and blooms more intense throughout the study period.     

Interannual variability of Aureococcus anophagefferens in Quantuck Bay, Long Island: natural test of the DON hypothesis

This study examined benthic and pelagic rate processes from the perspective of benthic dissolved organic matter (DOM) and its possible role in Aureococcus anophagefferens population dynamics. Sampling was conducted in Quantuck Bay, Long Island, New York, at three times in the summer of 2000 and two times in the summer of 2001. A. anophagefferens exhibited a large bloom between the May and July 2000 sample periods, but a smaller bloom was captured in the September 2000 sampling. Densities throughout 2001 were significantly lower than during 2000. There were few differences in most parameters measured between years, but the largest difference was the seasonal increase in both particulate (POM) and dissolved organic matter (DOM) during 2000 that was not observed during 2001. In particular, DOP accumulated the most, followed by DON and DOC, which resulted in significant seasonal decreases in the C:N:P ratios of the DOM pools. On the contrary, changes in elemental ratios of POM were not observed. The seasonal accumulation of DON appeared to be driven largely (50%) by the flux of DON from the benthos in 2000, but during 2001, all measured DON fluxes were into the sediment from the water column. This is consistent with the lack of accumulation during this year. There was little evidence for changes in microzooplankton grazing pressure between 2000 and 2001, and therefore the accumulation of DON and DOP during 2000 could have provided a competitive advantage to A. anophagefferens over other picoalgal species (e.g., Synechococcus) resulting in the significant blooms observed in 2000.     

A comparison of N and C uptake during brown tide (Aureococcus anophagefferens) blooms from two coastal bays on the east coast of the USA

Blooms of the brown tide pelagophyte, Aureococcus anophagefferens, have been reported in coastal bays along the east coast of the USA for nearly two decades. Blooms appear to be constrained to shallow bays that have low flushing rates, little riverine input and high salinities (e.g., >28). Nutrient enrichment and coastal eutrophication has been most frequently implicated as the cause of A. anophagefferens and other blooms in coastal bays. We compare N and C dynamics during two brown tide blooms, one in Quantuck Bay, on Long Island, NY in 2000, and the other in Chincoteague Bay, at Public Landing, MD in 2002, with a physically similar site in Chincoteague Bay that did not experience a bloom. We found that the primary forms of nitrogen (N) taken up during the bloom in Quantuck Bay were ammonium and dissolved free amino acids (DFAA) while the primary form of N fueling production at both sites in Chincoteague Bay was urea. At both Chincoteague sites, amino acid carbon (C) was taken up while urea C was not. Even though A. anophagefferens has the ability to take up organic C, during the bloom at Chincoteague Bay, photosynthetic uptake of bicarbonate was the dominant pathway of C acquisition by the >1.2 μm size fraction during the day. C uptake by cells <5.0 μm was insufficient to meet cellular C demand based on the measured N uptake rates and the C:N ratio of particulate material. While cells >1.2 μm did not take up much organic C during the day, smaller cells (>0.2 μm) did. Peptide hydrolysis appeared to play an important role in mobilizing organic matter in Quantuck Bay, where amino acids contributed substantially to N and C uptake, but not in Chincoteague Bay. Dissolved organic N (DON), dissolved organic C (DOC) concentrations and the DOC/DON ratio were higher and total dissolved inorganic N (DIN) concentrations were lower at the bloom site in Chincoteague Bay than at the nonbloom site in the same bay. We conclude that A. anophagefferens is capable of using a wide variety of N and C compounds, and that nutrient inputs, biotic interactions and the dominant recycling pathways determine which compounds are available and which metabolic pathways are active at a particular site.     

The influence of plankton composition and water quality on hard clam (Mercenaria mercenaria L.) populations across Long Island's south shore lagoon estuaries (New York, USA)

We conducted a two-year study to assess how plankton composition and water quality impacts the distribution, densities, condition, growth, biochemical composition and reproductive success of juvenile and adult Mercenaria mercenaria (L.) in Long Island's south shore estuaries (LISSE). Juvenile and adult hard clams were placed in suspended cages at 10 locations ranging from the ocean inlets to locations furthest from inlets in Shinnecock Bay (SB), the eastern-most barrier island estuary of LISSE, and Great South Bay (GSB), the western-most barrier island estuary of LISSE. Phytoplankton community composition, temperature, salinity, dissolved oxygen, and clam growth and condition were monitored bi-weekly. A benthic survey of M. mercenaria densities in both estuaries was also conducted. In both 2004 and 2005, juveniles in central bay locations had significantly faster growth rates, lower mortality rates, and higher lipid content relative to sites closest to the inlets. Adult hard clams closest to the Fire Island inlet also had significantly lower condition indexes compared to mid-bay stations and densities of wild M. mercenaria populations in both estuaries were lower near inlets compared to locations further from inlets. In addition to substantial spatial differences within each estuary, differences were also observed between the embayments as juvenile clams in SB grew approximately twice as fast as those in GSB and adults in SB had significantly greater condition indexes than clams in GSB. Instantaneous juvenile growth rates were highly correlated to temperatures below 24 °C (p < 0.0001) and were also significantly correlated with several indicators of suspended food quantity and food quality (centric diatoms, phytoplankton cells > 5 μm, and dinoflagellates (inverse correlation)) which co-varied independently of temperature. In sum, these results suggest tidal exchange in LISSE promotes a water quality regime (cold water, with low food concentration) which would reduce the growth of juvenile clams and the overall reproductive success of adult hard clams located near newly-formed ocean inlets. However, increased exchange for regions furthest from inlets could enhance juvenile clam growth rates by reducing summer peak temperatures (> 24 °C) and densities of poor food sources (dinoflagellates).     

Toxicity of Alexandrium lusitanicum to gastropod larvae is not caused by paralytic-shellfish-poisoning toxins

Laboratory grazing experiments compared ingestion of two subclones of the dinoflagellate Alexandrium lusitanicum by gastropod veliger larvae (Nassarius sp.). While the two prey subclones originated from the same monoclonal isolate of A. lusitanicum, one possessed the ability to produce paralytic-shellfish-poisoning toxins (PSTs), while the other did not. Ingestion rates on the two Alexandrium subclones were not significantly different over a range of prey concentrations (approximately 100–660 cells ml⁻¹), indicating that PSTs did not serve as a grazing deterrent for these larvae. However, ingestion rates on both subclones were low at the higher prey concentrations tested. Mortality of the predators also increased linearly with concentration of either subclone. These observations indicated that both A. lusitanicum subclones produced an unknown substance that inhibited and killed the grazers. Veliger mortality was not induced by culture filtrates or lysates, suggesting either that the substance was either highly labile or that contact with intact cells was required. Because toxic algae can produce multiple bioactive substances, experimental demonstrations of alleopathic effects of toxic species should not be assigned to known toxins without supporting evidence. In addition, the results show that the effectiveness of algal grazing deterrents can increase with cell concentration, which may have implications for bloom dynamics.     

The effect of thermal stress on Ostrini00a nubilalis HBN (Lepidoptera, Pyralidae)—I. The orle of the neurosecretory cells in the survival of diapausing larvae

1. 1.|The effect of short-term exposure (up to 12 h) to upper limiting temperature (45°C) on A-type medial neurosecretory cells (MNSC) of the protocerebrum of Ostrinia nubilalis larvae was studied.

2. 2.|For evaluation of the activity of MNSC, cytological parameters were used: quantity of the neurosecretory material, size of the neurosecretory granules, shape of the nuclei and size of the nucleoli.

3. 3.|After 1 h exposure to 45°C the activity of the MNSC was increased, after 3 h it was decreased, while after 6 h exposure to the same temperature some renewed increase in the activity was observed.

4. 4.|The significance of oscillatory changes in the activity of A-type MNSC in survival of the thermal stress of O. nubilalis diapausing larvae is discussed.

Spatial covariance in resources affects photosynthetic rate and water potential,but not the growth of Glechoma longituba fragments

Glechom longituba is a widespread clonal plant and usually experiences diverse patchiness of its growing sites. The hypothesis was tested that spatial covariance in resources differentially affects the growth and physiology of G. longituba. Plants were exposed to two patchy habitats where above- and below-ground resources were either positively or negatively associated, and to four control habitats. When equal amounts of resources were given, G. longituba fragments from two patchy habitats had similar biomass, root weight ratio, leaf weight ratio, fluorescence yield, specific petiole length, and specific root length; fragments under reciprocal patchiness significantly increased photosynthetic rate and decreased leaf water potential. Biomass of clones grown in patchy habitats was equal or less than that of counterparts from the control habitats, not supporting the notion that clonal growth is more advantageous in patchy habitats than in uniform habitats. In addition, no evidence was detected for spatial division of labour because biomass allocation to roots and leaves was similar in patchy habitats compared with the control habitats.